1. Field of the Inventions
The present inventions relate to an exhaust system and, more specifically, to an exhaust system for a marine engine that includes an exhaust control valve for adjusting the exhaust pressure of exhaust gases.
2. Description of the Related Art
Watercrafts, such as water jet propulsion boats, produce a propulsion force that is generated by jetting water rearwardly from its stem. Water is typically introduced through the bottom of a water jet propulsion boat to a jet pump. The jet pump discharges the water rearwardly to generate a propulsion force. Such water jet propulsion boats typically include an exhaust conduit. Exhausts gases discharged from the engine pass through the exhaust conduit towards an external location.
Japanese Patent Application No. HEI 11-157494 discloses exhaust conduits that have an exhaust control valve for adjusting the exhaust pressure of the exhaust gases to reduce the exhaust discharge noise and for affecting the engine output.
Water jet propulsion boats also typically have a water-lock positioned midway along the exhaust conduit. Exhaust gases pass through the water-lock before being discharged to the atmosphere. An exhaust control valve can be positioned within the exhaust conduit at a location downstream of the water-lock. The water jet propulsion boats can have a servo motor for driving the exhaust control valve and an engine control unit (“ECU”) for controlling the operation of the servo motor.
The exhaust control valve is often moveable between a closed position and an open position. The ECU moves the exhaust control valve towards its closed position when the engine operates at a low speed and moves the exhaust control valve towards its open position when the engine operates at a high speed.
Exhaust conduits extending between the water-lock and the engine often have two passages; one passage through which the exhaust gases flow and a cooling water passage through which cooling water flows. The cooling water passage can be formed around an outer surface of the exhaust gas passage. The exhaust gas passage and the cooling water passage can merge the exhaust gas flow and the cooling water flow together at some point upstream of the water-lock.
The exhaust gas/cooling water mixture then flows into the water-lock. The internal pressure of the water-lock can be relatively high when the exhaust valve is closed. The high pressure in the water-lock can cause reverse the flow of the cooling water, i.e. towards the engine. If the pressure of the water-lock causes a reverse flow of the cooling water, the cooling water may enter the engine and impair engine performance.
When the engine operates at a high rotational speed, the period(s) of the exhaust pulses of the exhaust gases are generally shorter and the amplitude of the exhaust pulses are less than the period(s) of exhaust pulses produced when the engine operates at medium and/or low rotational speeds. Thus, reverse flow of cooling water through the exhaust conduit is less likely to occur at high engine speeds as compared to low engine speeds. When the engine operates at mid-range or low rotational speeds, the period(s) of the exhaust pulses are longer and the amplitude of the pulses are relatively large thereby increasing the likelihood of having a reverse flow of cooling water. Accordingly, reverse flow of the cooling water through the exhaust conduit towards the engine can occur during typical engine operation.